UKIP have accused the European Parliament of “pure victimisation” after a report accusing the party of misspending funds was leaked to the press before they had even had a chance to read it.

The party accused the parliament of moving the goal posts after it claimed the pro-Brexit party had spent European funds on polling in key target seats before the 2015 General Election, and before the Brexit referendum.

The money was provided to UKIP’s European grouping the Alliance for Direct Democracy in Europe (ADDE), made up of Eurosceptic parties from across the continent.

The audit says that money was spent on “indirect financing of a national political party” and “a referendum campaign”, both of which are banned under EU law.

The report, revealed by Sky News, claims €500,615.55 was misspent by the ADDE, much of which was allegedly spent to fund polling in Great Grimsby and Thurrock, Rochester and Strood and Cardiff South and Penarth, as well as polling before this year’s referendum.

However, an ADDE spokesman has accused EU officials of changing the definition “expenditure supporting a political party”, and said the group would be taking the matter to court.

“The parliament administration has for months taken an aggressive and hostile attitude over the audit, amounting to nothing short of deliberate harassment,” he said.

“We have responded to their queries with a mass of information and explanation justifying our activities and expenditure. They have simply ignored our submissions and in several cases these submissions have been made repeatedly on their request.

“They have broadened ex-post the definition of ‘expenditure supporting a political party’ so widely as to deny us the right to undertake any activity which might be remotely interesting to ADDE members.”

The group’s spokesman added: “It has become increasingly apparent since Brexit that anything short of ‘group think’ is no longer tolerated within the European Union. Any deviation will see the rules changed and goalposts moved.

“Everything the ADDE has conducted is to provide a more coherent overview of the opinions expressed by the population that is represented by the members of the ADDE.

“We are therefore confident that our expenditures – with the exception of a few minor items – are fully eligible and compliant to EU regulations. They are also in line with fully accepted activities from other groups.